1. Field of the Invention
This invention relates to a vapor phase growth method for obtaining a gallium arsenide (GaAs) epitaxial layer having a highly uniform thickness with a good reproducibility.
2. Description of the Prior Art
The vapor phase growth method is a method for growing a thin film layer of a single crystal on a single crystal substrate and this method is widely used in the production of semiconductor elements. GaAs is used as a material of various semiconductor elements, such as field effect transisters and Gunn diodes. The method of the vapor phase growth of GaAs is important for the manufacture of these semiconductor elements. With recent demands for mass production of GaAs elements, improvement of the reliability of performances of semiconductor elements and the increase of integration in circuits, a technique of forming an epitaxial layer having an improved uniformity on a substrate of a large area with a good reproducibility has eagerly been desired. A method of the epitaxial growth of continuous multi-layer structures is ordinarily adopted for production of GaAs elements. In order to obtain such continuous multi-layer epitaxial growth elements at an enhanced yield, it is necessary to grow an epitaxial layer having a high thickness uniformity with a good reproducibility.
In most conventional methods for forming epitaxial layers of GaAs for semiconductor elements, the growth is carried out generally under diffusion limited conditions. In the growth carried out under diffusion limited conditions, the thickness distribution of the eptaxial layer formed on the crystal surface of the substrate is considerably influenced by growth conditions, such as the growth temperature, the flow rate of the feed gas, the molar fraction of AsCl.sub.3 and the temperature gradient in the vicinity of the substrate. Therefore, it is necessary to carry out the growth while controlling these growth conditions very precisely. Accordingly, in the conventional methods, the reproducibility is poor and it is very difficult to obtain an epitaxial layer having a high thickness uniformity. In the growth carried out under the above-mentioned conditions, since a boundary layer formed on the crystal surface of the substrate controls the thickness distribution of the epitaxial layer, it is very difficult to grow an epitaxial layer having a highly uniform thickness on the crystal surface of a substrate having a large area.
As another instance of the vapor phase growth method, there is known a method in which the growth is carried out under kinetically limited growth conditions, and according to this method, a substrate is placed in a uniform or constant growth temperature region. This growth method is advantageous over the growth conducted under diffusion limited conditions. The growth rate is hardly influenced even if the mole number of molecules in the reaction gas and flow rate of the gas are changed more or less. The boundary layer also has no influence on the thickness distribution and if the uniform growth temperature region is sufficiently long, an epitaxial layer having a uniform thickness can be formed on the crystal surface of a substrate having a large area. The growth in this region, however, is carried out at a temperature lower than the temperature adopted for the growth under diffusion limited conditions, and the degrees of supersaturation of the vapor phase is increased by lowering the growth temperature. Accordingly, a polycrystalline GaAs is deposited on the wall of the reaction tube upstream of the substrate crystal or in the vicinity thereof, with the result that the reproducibility among growth runs is poor and it is difficult to obtain an epitaxial layer having a highly uniform thickness.